Abstract

3D detectors are a novel variety of photodiode radiation detector, invented by Parker, Kenney and Segal (1997). Instead of having n- and p-type contacts on the front and back surfaces of a silicon substrate, like a standard photodiode, they have columns of doped material passing through the thickness of the silicon. This structure means that the detector can combine a reasonable substrate thickness with a very small electrode spacing, resulting in a low depletion voltage, fast charge collection and low charge sharing.
These detectors have a couple of promising applications. Their fast charge collection and low depletion voltage should make them very radiation-tolerant. So, they could be used for future particle physics experiments at the Super Large Hadron Collider (SLHC), where high levels of radiation damage are expected. Also, their low charge sharing means they could potentially improve X-ray diffraction measurements at synchrotrons such as Diamond Light Source. This would allow these experiments, for example, to determine the structures of biological molecules more accurately.
However, before 3D devices can be used in practical experiments, their design and fabrication must be optimised to ensure that reliable, high-performance detectors can be produced on a reasonably large scale. The aim of this thesis is to evaluate and understand the behaviour of a variety of 3D detectors using a combination of lab tests and computer simulations. Using these results, future fabrication runs can then be re-designed to improve their performance.
Firstly, the "Synopsys TCAD" simulation package was used to determine the optimum design for 3D detectors at the SLHC. It was found that the device behaviour depends strongly on the electrode spacing, and the choice of spacing requires a trade-off between different effects. Using a smaller spacing reduces the detector's operating voltage, and improves the charge collection efficiency by reducing carrier trapping. However, reducing the spacing also increases the capacitance, resulting in greater noise, and also increases the insensitive volume occupied by the columns. At SLHC radiation damage levels, the optimal electrode spacing was found to be 40-55 micrometres.
CNM (Centro Nacional de Microelectronica) in Barcelona have produced a set of "double sided" 3D detectors. The n- and p-type columns in these devices are etched from opposite sides of the substrate and do not pass through the full substrate thickness. Computer simulations show that these detectors should give similar performance to full-3D detectors. The main difference is that these devices have slower charge collection around their front and back surfaces. Basic electrical characterisation of the detectors showed that they have low depletion voltages. However, the guard ring current varied a great deal between detectors, though this was fixed by using better guard structures. Charge collection tests on these detectors using beta particles gave mixed results. A heavily-irradiated detector gave a relatively high collection signal, similar to the simulated value, which demonstrated the structure's radiation hardness. However, an unirradiated detector gave an unexpectedly low collection signal. This was perhaps due to poor coupling between this detector and the readout chip.
Three of these "double-sided" 3D detectors were bonded to Medipix2 pixel readout chips. These chips are specifically designed for X-ray detection, and can count individual photon hits. The detectors worked successfully, and initial lab tests demonstrated that they depleted extremely rapidly. The detectors were then tested in an X-ray beam at Diamond Light Source. These tests showed that the detectors have lower charge sharing than a standard planar photodiode. For example, 24% of the hits on a double-sided 3D detector at 22V were shared, compared to 40% on a planar detector at 100V.
A set of devices with a simplified "single-type-column" structure, fabricated by FBK-IRST in Trento, were also tested. Simulations showed that although this structure will have a low depletion voltage and fast electron collection, the hole collection will be slow. This will result in poorer behaviour than full- and double-sided 3D detectors. This was confirmed by lab tests, which showed that when the detector was coupled to fast readout electronics, the charge collection efficiency was reduced due to ballistic deficit.